Method of purifying metals and recovery of metal products therefrom



June 10, 1969 L J. DERHAM 3,449,117

METHOD OF PURIFiING METALS AND RECOVERY OF METAL PRODUCTS THEREF'ROM 1 Flled Sept. 22. 1965 Sheet or 14 FEG.1

ll i 2 INVENTOR Leslie Jock Derhorn v BY MW MMW ATTORNEYS June 10, J DER M METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22. 1965 v Sheet & of 14 I 1 INVENTOR Leslie Jock Derhum ATTORNEYS June 10, 1969 l J. DERHAM 3,449,117

METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22, 1965 Sheet 3 of 14 INVENTOR Leslie Jock Derhom ATTORNEYS June 10, 1969 J. DERHAM METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22. 1965 Sheet of 14 INVENTOR Leslie Jock Derhum BY M,

June 10, 1969 L. J. DERHAM 3,449,117

METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22, 1965 Sheet 5 of 14 lNVENTOR Leslie Jock Derhum ATTORNEYS June 10, 1969 J. DERHAM 3,449,117

. METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22. 19 65 Sheet 6 of 14 Impure Treatment MoltenZunc D vice or 92 Purified Devices ZincVupour Impure Zinc Vapour 88 Zinc l Blqst Furnace Condenser Zinc Residual Metal 208 Treatment Device or Devices INVENTOR Leslie Jack Derhom m ,aoxi g z g ATTORNEYS June 10, 1969 J. DERHAM METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Sheet Filed Sept. 22, 1965 INVENTOR Leslie Jack Derhom 7 BY MM ATTORNEYS June 10, 1969 1.. J. DERHAM METHOD OF PURIFYING METALS AND RECQVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22, 1965 Sheet INVENTOR Leslie Jock Derhom ATTORNEYS June 10, 1969 L. J. DERHAM 3,449,1 1 7 METHOD OF PURIFYING METALS,AND RECOVERY OF METAL PRODUCTS THEREFROM Filed Sept. 22. 1965 Sheet 9 of 14 INVENTOR Leslie Jock Derhom ATTORNEYS June 1969 L. J. DERHAM METHOD OF PURIFYING METALS AND RECQVERY OF IO of 14 METAL PRODUCTS THEREFROM Filed Sept. 22. 1965 Sheet II/////// I Fm mm N0 Om wow INVENTOR Leslie Jock Derhom BY x2144 I ATTORNEYS June 10, 1969 J. DERHAM METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Sheet [I of 14 Filed Sept. 22. 1965 INVENTOR Leslie Jock Derhom BY M W/%% ATTORNEYS June 1969 L. J. DERHAM METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM 74 11 1 9 4 3% t e e h S Filed Sept. 22, 1965 1 r 1 1 I I t r I 4 1 r I I I I I IIIIIIIIIII INVENTOR Leslie Jock Drhom mp wI ATTORNEYS June 10, 1969 1.. J. DERHAM METHQD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Sheet Filed Sept. 22, 1965 wO Nov & wmm

Q Q E mwm INVENTOR LeslieJock Derhom M W, ms f YMAQ ATTORNEYS June 10, 1969 L. J. DERHAM METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFRO M Sheet i of 14 FIG.

lNVE-NTOR Leslie Jack Derhom ATTORNEYS United States Patent 3,449,117 METHOD OF PURIFYING METALS AND RECOVERY OF METAL PRODUCTS THEREFROM Leslie Jack Derham, Avonmouth, Bristol, England, as-

signor to Imperial Smelting Corporation (N.S.C.) Limited, London, England, a British company Continuation-impart of application Ser. No. 308,134, Sept. 11, 1963. This application Sept. 22, 1965, Ser. No. 489,267 Claims priority, application Great Britain, Sept. 27, 1962, 36,763/62; Oct. 15, 1962, 38,975/62 Int. Cl. C22b /16, 19/04; C22d 7/00 US. Cl. 75-10 20 Claims ABSTRACT OF THE DISCLOSURE A method of recovering at least one volatilizable metal in an alloy of metals containing at least One metal with a boiling point higher than the boiling point of the metal to be recovered. A continuous upright column of a mixture of separate carbonaceous lumps and separate briquettes containing the alloy metal in finely divided form is passed by gravity through an electrothermically heated zone at a temperature sufiiciently high to melt, and then to distill, the metal to be recovered; but at a temperature suificiently low to retain the metal of higher boiling point in unvaporized form. The briquettes of finel divided metal alloy become spent residues by the time they reach the bottom of the column. The residues are separated from the carbonaceous lumps and removed from the system. The carbonaceous lumps are unconsumed and retain substantially their original form for reuse in the treatment of further briquettes of alloy metals of the same type.

Related applications, all filed simultaneously herewith, are Ser. Nos. 489,263, 489,264, 489,265, 489,266, and 489,268.

The present application is a continuation-in-part of my pending application Ser. No, 308,134 filed Sept. 11, 1963, and now abandoned.

This invention relates to method of purifying metals and the recovery of the purified metals as such, in molten or solid slab, or in solid powder or dust form; or as a compound, such as solid metal oxides. The invention relates more particularly to a purification system involving the selective vapourization and withdrawal of one or more metals and the selective retention and withdrawal of one or more unvapourized molten metals.

While the practice of the invention is applicable to a plurality of metals, and their alloys, more especially the lower melting point and the lower boiling point metals, so far as vapourization is concerned, it is proposed to emphasize below particularly the treatment of impure zinc; although calling attention also to other metals amenable to the procedure or procedures herein contemplated. The principal limitation is the ability of the equipment, the apparatus, in the system economically to withstand necessary wear and tear. The invention is practiced in a continuous, or substantially continuous, treatment operation.

ZINC

The chief Zinc ore is Zinc blende, or sphalerite, ZnS. Other ores include calamine, Zn SiO .H O; smithsonite, ZnCO zincite, ZnO; willemite Zn SiO and franklinite,

(Zn, Fe, Mn)O' (Fe, Nn) O Zinc produced pyrometallurgically and electrothermically with carbon as a reducing agent is sometimes contaminated with such major impurities as lead, iron, cadmium, tin, sulphur, arsenic, antimony, manganese, and copper, and such minor impurities as nickel, cobalt,

germanium, thallium, mercury, silver and gold. To obtain high purity zinc, it must, of course, be suitably refined.

The conventional method of purification is to boil the zinc metal by heating it in a retort or aboiler composed of superimposed trays down which the molten zinc flows. This retort has to be constructed of refractory material, the thermal resistance of which limits the rate of heat transfer, Any residual material in the retorts further reduces the rate of heat transfer. A further disadvantage is that the retorts or tray columns are susceptible to thermal shock.

It is known to boil zinc metal electrothermically by means of resistor heaters from which the heat is radiated to the zinc metal. Owing to the low emissivity of liquid zinc, however, most of the heat radiated from the resistor heaters is reflected by the liquid zinc back to the refrac tory roof of the furnace, which thereby becomes heated to a much higher temperature than the'zinc metal. This method of heat supply is therefore ineificient.

In accordance with the invention disadvantages of the kind enumerated are for the most part avoided. Zinc vapour is produced from metallic zinc by a new method, which is both simple in principle and highly efficient, and which can be carried out in apparatus that is both simple and durable. That method and apparatus involve a number of highly useful modifications for the production of zinc vapour from molten metallic zinc by means of heat generated by electric current.

The invention involves the production of relatively pure zinc vapour from zinc metal containing less volatile impurities, such as lead, by supplying sufiicient electrothermic heat to the molten impure zinc metal to volatilize a major portion of the zinc, leaving the less volatile metals with a small amount of residual molten zinc.

Another aspect of the invention is to obtain relatively pure zinc metal from zinc metal containing cadmium as an impurity, by supplying suflicient electrothermic heat to the molten impure zinc metal to volatilize part of the zinc and substantially all of the cadmium, the residual molten zinc thus obtained being substantially free from cadmium.

In the practice of the invention one is able to produce purified zinc metal, zinc dust of controlled particle size and of high metallic zinc content, and Zinc oxide.

The high electrical conductivity of molten zinc has hitherto made it impractical to volatilize molten zinc by supplying electrothermic heat as electric current passing through the molten zinc.

I have discovered that when molten zinc is distributed substantially evenly over a packed column of carbonaceous solids or lumps or briquettes, to which an electric current is applied, the mode of flow of the zinc is such that its high electrical conductivity does not result in such a high electrical conductance of the flowing zinc as to make electrothermal heating impractical.

The invention further contemplates a method of and apparatus for producing relatively pure zinc vapour from zinc metal containing less volatile impurities, such as lead and iron, in which the impure molten zinc is supplied to the top of a packed column of carbon lumps and electrothermic heat is applied, through electrodes at the top and the bottom of the column, directly to the molten zinc metal as it flows down the column, whereby most of the zinc is volatilized, while the small remainder of the zinc, containing the less volatile metallic impurities, is run off from the bottom of the column as zinc residual. Since the electrothermic heat is supplied directly to the molten zinc metal, the rate at which zinc is vaporized is closely related to the rate of supply of electrical energy. If the rate of supply of electrical energy is maintained constant, the rate of volatilization of zinc can likewise be maintained constant. With a constant rate of supply of molten zinc to the column, a constant ratio of zinc volatilized to zinc run off from the bottom of the column can be maintained. The zinc vapour, separated from its less volatile metal impurities, can be condensed to molten zinc. This condensation can be effected by conducting the zinc vapour to a condenser constructed of refractory material, such as silicon carbide, the surface area of the condenser being suflicient to cool the zinc vapour down to liquid zinc.

The invention also relates to the production of relatively pure metal vapours from metal alloys with other metals. One example of such a product is a zinc-iron alloy, commonly known as hard metal, that is recovered from the bottom of galvanizing pots. Other examples of such zinciferous materials are zinc alloys in which a main component, apart from zinc, is aluminum. In the process of this invention, the zinc alloy is first powdered by heating it to its hot-short temperature range, not far below its melting point. The fragmentation of the alloys is carried out by milling the alloy lumps in a rotary kiln with steel rods or steel balls, heat being applied to the kiln from an external source. The resulting metallic powder is then briquetted with wet or damp clay, together with a small amount of a carbonaceous material, such as anthracite coal; after which the briquettes are dried to harden or indurate them. During the drying out operation, moisture and hydrocarbons are driven from the metal-containing briquettes. This is advantageous because it facilitates the condensation of the metal, such as zinc, vapour. The vapour should be as free as possible from admixture with other gasses or vapours, particularly those that, like water vapour, can oxidize the metal (zinc) vapour. The metalcontaining briquettes thus produced, together with coke lumps, are introduced into an electrothermic furnace, where the zinc is distilled off. The zinc vapour may be condensed to zinc metal or converted to zinc dust or to zinc oxidepigment.

In order to improve further the purity of the zinc metal, the zinc vapour, before being passed to the condenser, may be conducted upwards through a refluxer column made of refractory material, such as silicon carbide. Some of the zinc metal condenses in this refluxer column and flows down the column countercurrent to the rising zinc vapour, whereby any non-volatilized metal impurities in the vapour are transferred to the downflowing liquid zinc. From the top of the column, the purified zinc vapour is then conducted to a condenser.

The invention also involves a method of and apparatus for obtaining relatively pure zinc metal from zinc metal containing a more volatile metal impurity, such as cadmium, in which the molten cadminm-containing zinc metal is supplied to the top of a packed column of coke lumps and electrothermic heat is supplied directly to this impure molten zinc metal at such a rate that some of the zinc is volatilized together with substantially all of the cadmium, while the rest of the zinc, substantially free from cadmium, is run off from the bottom of the column. The volatilized zinc and cadmium can be conducted directly to a condenser.

Alternatively, the volatilized zinc and cadmium ma be conducted upwards through a reflux column constructed of superimposed trays made of suitable refractory material, such as silicon carbide, the top of this column being cooled so that much of the upcoming zinc vapours are condensed; and the resulting condensed zinc metal flows down the reflux column countercurrently with the up coming vapours, only to be re-volatilized. The zinc vapours thereby became enriched in cadmium, and the zinc vapour leaving the top of the reflux column is conducted to a condenser.

In addtion the invention includes a method of and apparatus for obtaining high purity zinc metal from zinc containing impurities, such as cadmium, that are more volatile than zinc, and other impurities, such as lead and iron, that are less volatile than zinc. In this process the less volatile impurities are first removed by volatilizing part of the zinc and passing the resulting vapours through a refluxer column, and then a condenser, as already described. The condensed metal is then conducted to another electrothermic boiler, where part of the zinc metal is boiled off to remove the cadmium; the cadmium fraction containing some zinc is separately condensed and collected; and the balance of the zinc is run off from the base of the column as molten zinc freed from cadmium and also from other more volatile impurities.

The invention also involves a method of and apparatus for making zinc dust or powder in which electrothermic heat is supplied directly to molten zinc flowing down a packed column of carbon lumps or briquettes and the zinc vapour thus generated is supplied to a condensation system where its is rapidly chilled by a gas inert to the zinc. The inert gas is recirculated over and over again through the condensation system. The size of the dust particles produced is controlled by controlling the ratio of the volume of recirculated gas to the volume of zinc vapour entering the condensation system. Because of its ready availability and cheapness, nitrogen is usually the preferred inert gas; although argon or helium may be used.

As already indicated, the invention further includes a method of and apparatus for making zinc oxide in which electrothermic heat is supplied directly to molten zinc flowing down a packed column of carbon lumps and the zinc vapour thus generated is oxidized by admixture with a controlled volume of air.

LEAD

Most of our lead today is derived from one mineral, galena, PbS; and is recovered by lead-blast furnace smelting. Lead ores are generally of the argentiferous type, containing important amounts not only of silver, but also of gold and copper, which are recovered as by-products. In most ores the lead sulphide is usually associated with zinc sulphide, and either of the two may predominate. Smelting in blast furnaces is most often the best process because it allows recovery of the silver and gold and copper, and because the process is adapted to the handling of ores that are low in lead and high in impurities.

The lead ore, such as galena, concentrates are first roasted to remove most of the sulphur, changing lead sulphide, PbS, to a mixture of lead oxide, PhD, and lead sulphate, PbSO When the roasted product, still containing some sulphur is smelted, a small amount of matte is sometimes formed. It, together with some slag, collects at the bottom of the furnace. The matte has the valuable function of taking up most of the copper present in the concentrates; any copper not entering the matte goes into the molten lead metal, from which it is later removed.

The amount of matte formed, and consequently the distribution of the copper, can be controlled by the amount of sulphur left in the roast. The lead collects at the bottom of the furnace in the form of lead bullion,

and is removed through a liquid lead seal. The slag and matte are tapped together and run into settlers. The slag overflows from the settler, while the matte collects in the bottom of the settler.

The resulting lead bullion obtained from the blast furnace is refined and the silver and gold recovered from it; but, before that is done, other impurities, which interfere with the desilverisation process, must be removed. The chief impurities found in lead bullion are: silver, gold, copper, iron, antimony, nickel, cobalt, arsenic, tin, bismuth, sulphur, cadmium, and principally Zinc.

The present invention lends itself readily to the partial purification of the lead bullion, particularly in respect of those impurities which volatilize at a temperature below the boiling point (1750 C.) of the lead. This includes such impurities as cadmium, arsenic, sulphur and zinc. Those impurities are readily boiled off, while the remaining metal impurities, particularly those with melting points :below the boiling point (907 C.) of zinc, sink to the bottom of the furnace with the molten lead; and the partially purified lead bullion is then subjected to other processes of purification, with which the present invention is not concerned.

CADMIUM The commercial occurrence of cadmium is exclusively in combination with zinc ore, including complex ores of zinc, copper and lead. The normal cadmium content of zinc concentrates is less than 0.5%, although, in a few rare cases, it may run to 12%. Cadmium is therefore a 'by-product of zinc, lead and copper smelting. The byproduct results in several ways. The present invention is concerned, more especially, with the refining of zinc by distillation which yields a fraction of high cadmium content. The cadmium content of these cadmium byproducts may run from as little as 23% to 25% or more. The cadmium metal may be collected as powder, but usually is cast in molten form into bars or anodes, because the major use of cadmium is for plating.

The zinc produced by all the carbon reduction or smelting processes requires a refining step to produce zinc of the highest purity. A process widely used commercially is based on fractional distillation in reflux refining columns. A typical unit consists of two columns, followed by a single cadmium column. The columns serve to remove lead, iron and other high-boiling point metal impurities; whereas the cadmium column removes cadmium and other low-boiling point metal impurities.

The resulting cadmium may be subjected to the purification procedure of the present invention to eliminate the zinc and to produce high purity cadmium, which may be molded into desired shapes, or be converted to powder or to a compound, such as cadmium oxide.

ANTIMONY Antimony occurs in nature in both the free and combined states. The principal ore is stibnite, Sb S Another source of antimony is senarmontite, Sb O Antimony is quite frequently found associated with the ores of lead, zinc, copper, and silver. The free metal is prepared by reduction of antimony oxide with carbon. In the case of antimony sulphide, it is first roasted in air to give antimony tetraoxide, Sb O which is then reduced with carbon. The stable form of antimony is the metallic modification. It forms brittle, silvery white, rhombohedral crystals which melt at 630.5 C. and have a boiling point of 1380 C.

Unrefined antimony contains such impurities as sulphur, iron, arsenic, tin and sometimes copper, gold, lead, zinc and cadmium. Such impure antimony may be treated in accordance with the present invention. The sulphur, arsenic, zinc and cadmium impurities may be readily distilled off at a temperature 'below the boiling point but above the melting point of the antimony, but in the vicinity of the boiling point of the zinc. The molten antimony may flow to the bottom of the furnace and there 'be run off; or it may be separately distilled at or slightly above its own boiling point, while the higher boiling point metal impurities remain behind. The separately distilled antimony may be condsened and molded as such into any desired solid form; or it may be converted into solid powder or into a solid compound, such as antimony oxide.

These and other features of the invention will be better understood, it is believed, by referring to the accompanying drawings, taken in conjunction with the following description, in which:

FIG. 1 is a vertical partial section of a furnace, illustrative of a practice of the invention, on the line 11 of FIG. 2;

FIG. 2 is a vertical partial section of the same furnace on the line 22 of FIG. 1;

FIG. 3 is a vertical partial section on the line 33 of FIG. 2;

FIG. 4 is a horizontal section on the line 44 of FIG. 1;

FIG. 5 is a horizontal section on the line 5-5 of FIG. 2;

FIG. 6 is a horizontal section on the line 6-6 of FIG. 2;

FIG. 7 is a plan view of the top of the storage pot;

FIG. 8 is a plan view on line 88 of FIG. 2;

FIG. 9 is a diagrammatic view in elevation, with parts broken away, of the combination of a zinc 'blast furnace and a condenser with an electrothermic furnace of the invention;

FIG. 10 is a diagrammatic view in elevation,'with parts broken away, of an electrothermic furnace of the invention fed with solid pieces of metal;

FIG. 11 is a simple diagrammatic elevation of apparatus, also illustrative of a practice of the invention, employable in the production of purified zinc metal and the like;

FIG. 12 is a diagrammatic view in elevation, with parts broken away, of apparatus, illustrative of a practice of the invention, used to produce zinc dust or powder;

FIG. 13 is an abbreviated diagrammatic view of a different embodiment of the inert gas compressor arrangement of FIG. 12;

FIG. 14 is a diagrammatic elevation, partly in section, of apparatus, illustrative of a practice of the invention, that may be used, for example, to produce high purity zinc oxide and the like;

FIG. 15 is a sectional elevation of an electrothermic furnace, showing a first as well as a second reflux condensing column, with a condenser interposed between and connecting them, and a canister (or condenser) for cadmium and the like connecting the top portion of the second reflux column;

FIG. 16 is a vertical section on line 16-16 of FIG. 15; FIG. 17 is a sectional elevation on the line 1717 of FIG. 15;

FIG. 18 is an elevation, partly in section, of an eX- tension of FIG. 15, showing the substitution of a zinc cadmium condenser and refluxing column for the cadmium canister (or condenser) of FIG. 15;

FIG. 19 is a diagrammatic side elevation, partly in section, of yet another apparatus, illustrative of a practice of the invention, particularly useful for the separation of one or more metallic constituents from an alloy of metals;

FIG. 20 is a sectional elevation of line 20-20 of FIG. 19; and

FIG. 21 is an abbreviated composite representation, in the nature of a flow sheet, of the method and apparatus of the invention by specific references to the foregoing FIGS. 1-20.

FIGS. 1-8

Referring more particularly to FIGS. 1-8, the apparatus shown includes an electrothermic furnace 10 in which the vaporization of a metal or metals is carried out, essentially in an upright shaft 12, with an inner layer 14 of a heat-insulating refractory brick, an intermediate layer 16 of mica to provide electrical insulation, and an outer layer 18 of highly insulating brick. There is an outlet 20 in the upper portion of the shaft for the escape of metal vapour, such as zinc vapour. The shaft is packed with sized coke, coke lumps, carbonaceous briquettes, or similar carbonaceous material 22 to form an upright, preferably vertical, column. A graphite electrode 24 extends deeply into the top portion of the coke column, through a side wall of the shaft.

At the bottom of the coke column is another graphite electrode 26, which serves as a bottom tray to collect the residual unvolatilized portion of the metal or metals treated in the furnace, such as zinc, lead, antimony, cadmium, etc. In the case of zinc, for example, this residual portion, which contains the less volatile metallic impurities, can be run off periodically through a taphole 28 in the bottom portion of the furnace. This taphole is kept closed, except during tapping periods; or it may be kept open, for example, by a partially submerged removable bafile 30, which allows a continuous outflow of molten metal. The lower end of the baffle extends below taphole 28, but above the top surface of electrode tray 26, to provide a low narrow passageway 32 for the flow of molten metal or metals thereunder from the electrode tray up to and out of the taphole. A cleanout port 40, normally filled with a removable brickwork panel 42, is provided in a suitable position, preferably directly opposite taphole 28, to enable any blockage on the electrode tray or in the taphole to be cleared.

A similar cleanout port 50, normally filled with a removable brickwork panel 52, is provided in a suitable position, preferably directly opposite the vapour outlet or otftake 20, that enables a tool to be inserted therethrough for cleaning purposes.

The furnace is surmounted by a carefully insulated top 54.

To continue with a description of the apparatus, in terms of zinc purification, as an illustrative example, the impure zinc to be charged to the furnace is first melted in a storage pot 56, from which four spaced tapered graphite crucibles 58 in the top 54, of furnace are kept supplied with molten zinc. Each of the crucibles 58 (FIGS. 3 and 8) has in its base a centrally located small orifice 60 through which the molten zinc can pass into a similar but slightly larger tapered crucible 62; into the upper part of which the crucible 58 snugly fits. In the base of each of crucibles 62 is a centrally located orifice 64 (FIG. 3) somewhat larger than each of the orifices 60. Molten zinc fiows at a steady rate through each of these orifices 64, and thus the zinc input to the furnace is distributed at four symmetrically placed points over the entire cross-sectional area of the furnace. The diameter of the orifices 60 is such that the desired rate of molten zinc flow into the furnace can be attained with the zinc in the crucibles 58 maintained at a convenient level 65, between full and half full, while the diameter of the orifices 64 is such that molten zinc flows through at the desired rate with only a small hydrostatic head maintained in crucibles 62. Spaced crucibles 62 fit snugly in their corresponding holes in the top 54 of the furnace; and, since crucibles 58 in turn fit snugly in crucibles 62, ingress of outside oxidizing air into the electrothermic furnace is inhibited. Due to the spacing of the crucibles the molten zinc is dropped onto and substantially evenly spread across the top of the column of coke 22. The zinc threads its way over and around the pieces of coke, as it pcrcolates and trickles by gravity downwardly through the interconnecting interstices between the pieces of coke.

Any zinc oxide dross originally present on the body of molten zinc in storage pot 56 is skimmed off from time to time; and any dross subsequently formed by oxidation in crucibles 58 remains floating on top of the body of molten zinc contained therein, from which it may be skimmed from time to time, so that none of the dross can enter crucibles 62 where, since there is no access of air, no further oxide dross can form. Consequently, the molten zinc entering the furnace is substantially free from oxide dross. Further, with this method of introducing molten zinc into the furnace, one of the crucibles 58 can be replaced by another without allowing access of air to the furnace.

The residual unvolatilized molten zinc 66 at the bottom of the column, containing the less veloatile met-a1 impurities, is periodically or continuously removed through 8 taphole 28 into a chute or trough through which it flows and is collected in a pot 72.

Upper electrode 24 (*FIG. 2) connects with an electrical power line and lower electrode tray 26 connects with an electrical power line 82, both lines coming from a power source, director alternating current, not shown. The power input into the furnace is controlled by means of a voltage regulator 84.

Initially, it is the coke lumps which determine the electrical resistance characteristics of the column, but these are modified considerably when molten zinc flows over and down the column. The coke lumps are soaked with the passing molten zinc. The molten zinc tends to form zinc films or droplets on the surfaces of the coke lumps. Molten zinc probably seeps into cracks, crevices, channels, indentations and the like inevitably present in the coke lumps, often by capillary attraction. Heat is generated electrically largely in those zinc wetted areas; and, by virtue of this fact, the heat is used at high efficiency, having no thermal barriers to overcome. Efficient boiling is further promoted by the method used to distribute the molten zinc substantially evenly over and across the cross-sectional area of the top of the column.

Example I As an example, the furnace 10 used has a shaft 12 of square cross-section, 22 inches by 22 inches, and the distance between the inner tips of electrode 24 and electrode 26 is 8 /2 feet. The shaft was almost filled with metallurgical coke of a sizing between inch and 1 /2 inch.

The furnace was fed with impure molten zinc from storage pot 56 at the rate of 9 pounds per minute, which was equally distributed through the four crucible orifices 60 symmetrically disposed at a radial distance of 6 inches from the centre of the furnace top. A controlled rate of feed of 2 /4 pounds of molten zinc per minute through each orifice was attained by fitting, to form orifices 60, a silica jet of 1.86 millimetre bore into the base of each of the graphite crucibles 58. The orifices 64 in crucibles 62 are inch diameter.

Zinc, containing as its main impurity 1.2% lead, was fed into the furnace at the rate of 540 lb. per hour, of which 505 lb. were volatilized and dischanged through outlet 20 near the top of the furnace as high purity zinc; and 35 lb. were run otf from the bottom of the furnace through chute 70 as residual zinc containing 16% lead. The power supply to the furnace was 180 kilowatts, volts and 1500 amperes. The power consumed was thus 800 kWh. per ton of zinc volatilized; the conductance was 12.5 mhos.

Similar results were obtained with zinc of low lead content as feed metal. Pyrometallurgically produced zinc usually contains lead as its main impurity, with some iron and cadmium; other impurities usually being present in only very small concentrations. When zinc of this type is boiled in an electrothermic furnace according to the invention, the conductance of the charge comes within the range that makes electrothermic heating practicable.

The operator of the furnace adjusts the voltage regulator until a desired predetermined temperature of the column is reached. That optimum temperature is then maintained. Since the operation is a continuous one, once the optimum temperature point or range is established, there is little more for the operator to do, except to make sure occasionally that those conditions prevail. When everything is going properly, the furnace operates at just about capacity; and discharges metal, either as vapour or molten residual, as rapidly as impure metal is fed to the furnace.

Although all grades of primary zinc behave satisfactorily in this respect, the impurities in some types of secondary zinc alter the properties of the molten metal In such a way as to give too high a conductance; this effect is associated, for example, with the presence of alumnium. When an attempt was made to distil some secondary zinc aluminum alloy, containing about 4 percent aluminum, a satisfactory distallation could not be carried out. A trial was then made with a mixture of equal parts of primary zinc and secondary zinc alloy, this mixture containing about 2% aluminum, in the same furnace as used for Example 1. With 30 volts, the current was 2100 amperes, this giving a power of only 63 kilowatts, the conductance of the charge having risen to 70 mhos, nearly six times as large as was 'found in the same furnace with primary zinc as the feed. In other words, the power of 63 kilowatts is not satisfactory because it is necessary to obtain a high power input to the furnace coupled with a current low enough to be compatible with the currentcarrying capacity of the remainder of the electrical equipment. Thus, there'is a lower limit to the resistance of the furnace, below which the apparatus will not function.

A current of 2,100 amperes is near the safe maximum current-carrying capacity of the equipment, but the power input of 63 kilowatts is not suflicient to ensure a suitably high rate of metal distillation. Thus, if the resistance of the furnace drops to too low a level, operation becomes impractical and may be impossible. [I have however found that with molten zinc flowing over the carbon lumps a reasonably high resistance can be obtained to effect the desire volatilization.

As already indicated the purified zinc vapour may be condensed to molten zinc by known methods, the zinc thus produced being substantially free of the less volatile metallic impuritites. The zinc vapour may also be converted to zinc dust or powder by rapid chilling. To control the size of the dust particles, it is advantageous to circulate an inert gas under positive pressure around the condensing system. The zinc vapour may also be used for the production of zinc compounds, such as zinc oxide.

FIG. 9

Much of the worlds zinc is produced pyrometallurgically, that is by smelting. Among the important techinques employed is that of the use of a zinc blast furnace. FIG. 9 illustrates the use of such a furnace in conjunction with the present invention.

A conventional zinc blast furnace (or other smelting furnace) 86 has a Zinc vapour outlet '87 in its upper portion connecting a condenser 88, such as a lead splash condenser system, for the accumulation of a body of molten zinc 89. The condenser is provided with a valved gas escape conduit 90; a partially submerged discharge baflle 91; a downwardly declining molten zinc flow conduit 92, the discharge end 93 of which connects with the central top portion of an electrothermic furnace (successive electrothermic furnaces, essentially the same in construction, will be identified with the numeral 10, to which is added a prime digit. They are operated in substantially the same manner).

As before, the impure molten zinc is dropped onto, and is spread laterally over, carbon lumps 22 forming column 12. Also, as before, a small amount of residual molten metal-to-be-refined 66 accumulates in the bottom portion of the furnace and escapes past submerged baffle 30 through trough or conduit 70 into receiving tank or container 72.

The bulk of the impure molten metal (such as zinc) escapes from the heated furnace chamber through outlet conduit into any one of the treating devices to be described below, to be condensed as purified, or at least partially purified, molten metal; as metal dust or powder; or as metal oxide; or as a still further purified metal product.

FIG. 10

While the impure metal-to-be-refined may be, and preferably is, when convenient, fed into the electrothermic furnace in molten form, it may also be fed thereto in solid form. Apparatus for such a practice is shown in FIG. 10.

To this end, the center of roof 200 of electrothermic furnace 10" is provided with a charging hopper 202, which is closed, except during charging, by a bell 204. The charging device may be of the well-known conventional type. Finely devided powder, particles, pieces or chunks 206, 208, 210, etc. of the solid impure metal, such as zinc-to-be-refined, is fed intermittently through the hopper onto the sloping'top of column 12. of car-bon lumps 22. Due to the intense heat generated within the carbon lump column, the pieces of impure metal are melted. The resulting impure molten metal spreads laterally over the cross section of the column and percolates or trickles by gravity downwardly among, and in intimate contact with, the carbon lumps.

As before, the resulting metal (zinc) vapours rise to escape from the upper portion of the electrothermic furnace through vapour outlet 20 into a suitable treating device or devices, such as a condenser, a powder or a metal oxide forming arrangement; or indeed to one or more reflux condensing columns for additional purification. The molten metal residuals 66, on the other hand, escape from the lower portion of the furnace through conduit 70 into collector 72.

So far as the feeding of solid impure metal to the electrothermic furnace is concerned, I am aware that some practical operating difiiculties may be encountered. The heat to melt the solid zinc, to bring it up to its boiling point and to evaporate it, means that the heat requirements of the furnace must be increased. Also, there is the danger that if sufficient heat is not available, the whole top portion of the furnace may become blocked by a plug of solid zinc metal. Also, there may be a tendency for zinc vapour to condense on the cooler molten metal at the upper portion of the furnace and to run back through the column; although a beneficial refluxing action then takes place. Heating problems may be minimized, at least in part, by pre-heating the impure zinc, for example. To this end the particles or pieces of zinc may be placed in the charging hopper and bell well in advance of their discharge into the electrothermic furnace. Heat from the furnace, above the boiling point of zinc (907 C.), is then employed to pre-heat the charge before it is dumped onto the carbon lump column in the furnace shaft. While the pre-theating step will fall short of the temperature of molten zinc (419 C.), it approaches that temperature; and to that extent facilitates the treatment operation in the carbon lump column.

FIG. 11

As indicated above, the purified, or at least partially purified, metal vapour (such as zinc) coming from the electrothermic furnace may be variously treated, depending upon the final product desired, as well as the quality of that product.

The apparatus shown in FIG. 11 includes an electrothermic furnace 10 (such as that described, the furnace being electrothermatically operated in a manner similar to that described above, with regard to FIGS. 18). The present embodiment shows zinc vapour outlet 20 communicating with a conventional or other condenser 280. It is provided at its furthermost end with a molten zinc sealed discharged conduit 282 to deposit the condensed molten Zinc in a collecting vessel or mold 284. The uppermost part of, the condenser is provided with a valved exhaust conduit 286 for the escape of any extraneous gases that may be present in the condenser or which may have come over to the condenser from electrothermic furnace 10.

The apparatus is particularly useful in the production of molten zinc which dos not contain a substantial amount of metal impurities, such as cadmium, more volatilizable than the zinc itself. On the other hand, tmetal impurities, such as lead, present in the molten zinc, which are less volatilizable than the Zinc, find their way down the column of carbonaceous lumps in the furnace to its bottom. Here the resulting residual zinc 66, mixed with the less volatilizable metals, passes through liquid sealed conduit or trough to a receiving vessel 72.

FIGS. 12-13 To make zinc dust or powder, attention is directed to FIGS. 12-13. It consists of a furnace 10, as above described. The vapour outlet 20 connects with a sheet metal vessel which serves as a main condenser. The sheet metal walls of the vessel readily radiate heat, which helps to cool the interior space and the interior contents of the vessel. Circumferentially and symmetrically placed around the vapour inlet 20 to the condenser are a plurality, preferably four or more, of spaced nozzle or jets 102 through which recycled inert gas is blown at a controlled velocity. These jets are directed obliquely toward each other so that the streamlets of inert gas, which are not substantially hotter than ambient temperature, impinge on and into the stream of zinc vapour issuing from the outlet (inlet) 20 only a short distance, preferably less than two feet, inside the condenser; and thereby break up the incoming stream of zinc vapour into a great multitude of tiny globules or particles of molten zinc, which are instantly and widely dispersed throughout the condenser. The individual particles are instantaneously chilled by the cooler inert gas to a temperature at which substantially all of the zinc vapour is condensed to a solid fine dust. It is important that the nozzles or jets be placed in a position to impinge a plurality of spaced jets of the inert gas symmetrically and circumferentially onto and into the cloud or main stream of zinc vapour entering the condenser through inlet 20.

The upper part of the main condenser 100 is rectangular in cross-section. The lower parts of two halves of the condenser are tapered to form a pair of hoppers 103 in which most of the zinc dust is collected. These hoppers are sealed at their bottom by removable dust seals 104. Precipitated dust may be withdrawn from the bottoms of the hoppers from time to time. The top of condenser 100 is provided with a plurality of spaced pressure release caps 106.

The furnace gases, still containing zinc dust, leave condenser 100 at a temperature of about 300 C. and enter a forked conduit 108, extending from the top portion of the condenser to a pair of spaced baflled settling tanks 112, the bottom of each tank being in the form of a single hopper with a sealed damper 113 for periodic removal of dust. The tanks are formed of sheet metal to facilitate loss of heat.

A forked conduit 114 connects the tops of the two settling tanks with each other. Each tank is provided with a centrally located depending baflie 118, extending from the top of the tanks to almost the bottom of the tanks, so that the gases and dust passing under the baffles must follow a 180 change in course, as indicated by arrows 120. This causes the gases and dust to impinge upon the baffies as well as the walls of the tanks; and thus helps to precipitate newly formed dust particles onto the bottoms of the settling tanks.

Forked conduit 114 has a branch 122 at its upper end which connects with a polyethylene sleeve or balloon 124; in the form of an expansion chamber, with a nonreturn valve 128 at its free end. This expansion chamber copes with slight fluctuations of pressure by inflating or deflating, and the whole system can be kept under a small positive pressure of about, for example, 1 inch water gauge.

The second of the two settling tanks 112 is provided with a forked conduit 130 at its upper end, which connects with the top of a third bafl led settling tank 134. The bottom portion of the tank is tapered to form a hopper, in turn provided with a sealed damper for periodic removal of zinc dust. Like the other settling tanks, tank 134 is provided with a centrally located depending bafiie 136 extending from its top to near its bottom. Gases and dust escaping from the second settling tank 112 pass downwardly and under the baflie 136, through the small space below the baffle, and make a 180 change in course, as indicated by arrow 138. Impingement of the gases and dust against the baffle, and against the side walls of the settling tank accelerates deposition of the dust into the bottom of the tank. As before, the walls of this tank are formed of sheet metal to facilitate loss of heat. The gases in this third settling tank may become cooled to about 150 C.

From this settling tank 134 the gases and remaining suspended dust particles pass through a conduit 140 into a cyclone 142, where more of the fine dust settles and may be removed from the bottom of the cyclone through a sealed damper 143. The gases, still dust laden, pass through a conduit 144, with a damper or valve 146, which connects the top of the cyclone with the bottom of a bag house 148. The bag house contains a plurality of depending spaced long porous filtering bags extending from near the bottom of the bag house to near the top of the bag house. The dust laden gases from the cyclone pass into a relatively deep distributing manifold 152, to the top of which the bottoms of the filtering bags are attached. Collecting bags 153 are attachable to the bottom of the distributing manifold. They catch and retain falling dust particles.

The fine dust particles settle downwardly in and against the filtering bags, as well as in the manifold. The long bags are shaken from time to time to loosen and drop adhering dust particles to and through the manifold below, to collecting bags 153.

The gases escape laterally through the pores of the high filtering bags and leave the top of the bag house through conduit 154. That conduit connects with a power driven fan or variable speed blower 156. A short free ended conduit 158, with a valve 160, connects with the suction side of the fan or blower. The function of the valved conduit is to permit entry of regulated amounts of free air into the system from time to time, as required, to facilitate formation of the dust; as will be described below.

A circulatory main return conduit 164, with a valve 166, extends from the fan or blower to the immediate vicinity of the outlet (inlet) conduit 20 connecting the furnace with condenser 100. The blower is fitted with a by-pass 167 connecting its suction outlet with its pressure outlet, this by-pass having a valve so that the total volume of gas recirculated through the jets 102 may be varied by cycling gas through the fan rather than through line 164.

The circulatory return conduit terminates in the gas discharge or ato-mizing device 101, of any appropriate design, such as a hollow annular chamber, to receive and distribute relatively cool circulated inert gas to the plurality of circumferentially spaced jet orifices or jets 102 for the discharge of a plurality of small jets of gas obliquely onto and into the stream of metal vapour escaping from outlet (inlet) 20 into condenser 100.

Main return conduit 164 is provided with a number of branches, such as 172, with a valve 174, which connects the upper end of forked conduit 108; thus permitting return of circulated inert gas into condenser 100 and into .the first of bafiled settling tanks 112. Another branch conduit, such as 176, with a valve 178, makes a direct connection with the top of condenser 100.

Special provision is made for means to purge the system just described of air, to replace the air with inert gas, such as nitrogen (helium, argon), and to keep the entire system under positive pressure, somewhat higher than atmospheric, in order to prevent ingress of outside oxidizing air, and to replace such inert gas as happens to leak from the system with substitute amounts of inert gas.

The means include an open ended conduit 180, with a valve 182, which connects circulatory conduit 164 with 

